The answer is simple. you do not want to ruin the moment by finding yourself kissing your significant other inside your bedroom or in a park. You just wanna fantasize that you are in some wonderland/Dreamland, and its just the two of you. you just want the darkness to prevail so that you can take yourself out of the physical location you are presently in, and let the mind rest and forget about the present scenario so that you can get yourself to your desired dreamland with your significant other. Jay Nepal!